Heat Loss Survey: What It Reveals About Your Home (Real London Case Study)

If your boiler is on its way out, or you’re thinking about a heat pump, or a room in your house never quite gets warm in January — a heat loss survey is the piece of work that gives you real answers instead of guesses.

Most quotes in our industry skip this step. An installer eyeballs the radiators, asks how many bedrooms you’ve got, and writes down a boiler size. That’s how houses end up with 30 kW combis when they only need 18, or with a “new” system that still leaves the kitchen cold.

A proper heat loss survey is different. It measures the building room by room, calculates exactly how much heat is escaping through walls, windows and floors, and tells you whether each radiator can actually meet that demand. We use the BS EN 12831 methodology — the same European standard used for heat pump grant applications and commercial heating design.

Below is a real survey we carried out recently on a Victorian terrace in SW12 (we’ve removed the customer details). It’s a good example because the property is typical of the South London housing stock we work in every day — pre-2000 build, sash windows, suspended timber floors, dormer loft conversion. The kind of house where the heating “sort of works” but the back bedroom is always cold and the gas bill is always painful.

The headline numbers

That 119 W/m² figure is the one to focus on. It tells you how leaky the building fabric is, independent of how big the house is. A modern, well-insulated home will sit around 30–50 W/m². A typical late-Victorian terrace with single-skin masonry, original sash windows and an uninsulated suspended floor will sit between 90 and 130. Anything above 120 is genuinely poor and means you’re paying to heat the street.

This matters because the size of boiler or heat pump you actually need is driven by this number, not by a rule of thumb based on bedroom count.

The three types of boiler — quick recap

Combi — heats your hot water on demand from the cold mains. No hot water cylinder, no loft tank. Best for flats, terraces and small-to-medium homes with one bathroom and good mains pressure. (See our boiler installation service.)

System — heats a separate hot water cylinder in the airing cupboard. The boiler contains the pump and expansion vessel, but you don’t need a cold water tank in the loft. Best for larger family homes, properties with two or more bathrooms, or homes with poor mains pressure.

Regular (also called conventional or open-vent) — works with both a hot water cylinder and a cold water feed tank up in the loft. This is the older setup. Best kept where the property is already plumbed for it.

Boiler size is measured in kilowatts (kW), not litres. A 24 kW combi puts out 24,000 watts at full chat. The right kW for your home depends on your heat loss and your hot water demand — and bigger is not always better. Over-sized boilers short-cycle, run less efficiently, and wear out sooner.

How radiator sizing actually works

This is the bit most homeowners are never told, and it’s the reason so many radiators “underperform” without anything being broken.

Every radiator on the market has a published heat output — say, 1,500 watts. But that figure is measured at a specific test condition called ΔT 50 (delta-T 50), meaning the average water temperature inside the radiator is 50 °C hotter than the room. That assumes the boiler is running at a flow temperature of around 75 °C.

Three things affect what your radiator actually delivers:

• Flow temperature — the temperature of the water leaving the boiler. Older non-condensing boilers ran at 70–80 °C. Modern condensing boilers are most efficient at 55–65 °C. Heat pumps run cooler still, at 40–55 °C.
• Delta-T (ΔT) — the gap between the water inside the radiator and the air in the room. The bigger the gap, the more heat thrown out. A radiator running at ΔT 25 (typical heat-pump scenario) gives only about 40% of its rated output.
• Radiator type — K1, K2 or K3, shown below.

What we found, room by room

The survey covered eleven rooms across three floors. Each one gets a heat demand figure (how many watts it needs to lose to stay at design temperature on a –1.7 °C day) and a verdict on whether the existing radiator can actually deliver that.

Ground floor

The lounge is a typical front reception with a bay window and original sash windows. The high air change rate is largely down to draught around the sashes. The existing radiator is doing roughly half the work the room actually needs.

The kitchen is the largest room in the property by some margin, and the radiator is the same size you’d put in a small bedroom. Either replace with a K3, or split the heat across two emitters.

First floor

Second floor (loft conversion)

What the building fabric is doing

The survey also captures the U-values of every external element. U-value measures how readily heat passes through a material — the lower the number, the better the insulation.

For context, current Building Regulations expect 0.18 for walls and 0.15 for roofs in new builds. So the roof here is conducting heat about 20 times more readily than a modern equivalent.

The point isn’t that the property needs to be rebuilt — that’s not realistic for most of South London’s housing stock. The point is that fabric upgrades and heating upgrades should be considered together. Insulating the loft, dry-lining the worst external walls, and improving draught-proofing on the sash windows would knock several kilowatts off the heat loss figure, which in turn changes the size of boiler or heat pump that’s appropriate.

What the survey changes about the recommendation

Without this data, a typical “look around” quote on this property would have probably specified a 30 kW combi or a 24 kW system boiler with the existing radiators kept in place. The customer would have written a cheque, the engineer would have been gone in a day, and three rooms would still be cold next winter.

With the data, the conversation changes:

1. The lounge, kitchen, and loft master need radiator upgrades before any new heat source goes in.
2. The bathroom and shower room need heat emitters specified properly — not assumed to “work because they’re small.”
3. A heat pump is technically possible but currently impractical without fabric upgrades or a wholesale radiator overhaul. At heat-pump flow temperatures, only 3 of the 11 rooms would be served adequately by the existing emitters.
4. A modern condensing combi or system boiler is well within reach, sized to the calculated heat loss (around 19 kW for space heating, plus headroom for hot water demand).

Should you get a heat loss survey?

You don’t need one for every job. If you’re doing a like-for-like boiler swap and your existing system has worked reliably for years, the cost-benefit isn’t there. But if any of the following apply, it’s worth the survey:

• You’re considering a heat pump, now or in the next few years
• You’ve got a room that won’t warm up in winter
• Your gas bills feel disproportionately high
• You’re planning an extension or loft conversion
• Your boiler is over 12 years old
• You’ve added a bathroom and the hot water can’t keep up

Heat loss survey — frequently asked questions